There are various types of metal reducing processes for producing reduced iron or alloy iron. Among these, there is a process using powder of a metal oxide as a feedstock, producing spherical pellets, and reducing these at a high temperature. As examples of this type of process, there are shaft type hydrogen gas reducing furnaces, rotary kiln reducing furnaces, rotary hearth reducing furnaces, etc. The green pellets used in shaft type hydrogen gas reducing furnaces are obtained by forming powder ore into grains and reduced by hydrogen gas. On the other hand, in rotary kiln reducing furnaces or rotary hearth reducing furnaces, heat is supplied from the reducing furnace and the reduction reaction is performed by the carbon mixed in the green pellets. That is, in a rotary kiln reducing furnace or rotary hearth reducing furnace, pellets comprised of the carbon of coal, coke, etc. mixed with the metal oxide powders are used. These processes can use inexpensive coal etc., so have caught attention as economical methods of production of reduced iron. Further, they are being used as high productivity processes in rotary hearth reducing furnaces.
A rotary kiln is a furnace comprised of a rotating cylinder having a diameter of 2 to 5 m and length of 30 to 80 m. The cylinder is made of steel and is lined with refractories. The furnace temperature is 300 to 600° C. at the feed part and about 1100° C. at the exit. The green pellets fed are heated over about 6 hours to about 1100° C. At that temperature, the carbon and metal oxide in the green pellets react to produce carbon monoxide and metal and form the reduced pellets. The reduced pellets are discharged from the kiln and cooled. After this, they are used as the feedstock for electric furnaces or blast furnaces.
A rotary hearth reducing furnace is a reducing furnace of a type having a disk-shaped hearth of refractories with a cutaway center rotating on rails at a constant speed under a fixed ceiling and side walls of refractories (hereinafter referred to as a “rotary furnace”). The diameter of the hearth of the rotary furnace is 10 to 50 meters and the width of the hearth 2 to 6 meters. The green pellets are fed to be uniformly spread on the hearth of the rotary furnace. The hearth rotates and moves the parts of the furnace, that is, the feed part, the heating zone, the reducing zone, and the discharging part, along with the green pellets. The green pellets are charged into the about 1000° C. high temperature feed part. Next, they are heated at the heating zone by radiation from the high temperature gas to about 1200° C. or more, then the carbon and metal oxide in the green pellets react at the reducing zone, whereby reduced metal is produced. In a rotary hearth reducing furnace, since the heating is quick, the reaction ends in 7 to 20 minutes. The reduced pellets are ejected from the furnace and cooled, then used as feedstock for electric furnaces or blast furnaces.
Further, in a rotary furnace, since the green pellets are placed stationarily on the hearth, there is the advantage that the pellets are resistant to crumbling in the furnace. As a result, there are the strong points that there are little problems of powderized feed sticking to the refractories and the yield of the nuggets is high. Further, there are the advantages that the productivity is high and an inexpensive or coal-based reducing agent or powder feed can be used.
As explained above, in a rotary furnace, the green pellets comprised of a powder feed including metal oxide and carbon shaped into grains is spread on the rotary hearth where it is reduced by heating. The green pellets are placed relatively stationarily on the hearth. As a result, easy to handle pellet-shaped reduced metal is obtained. If powder agglomerates like pellets, the contact between the metal oxide and carbon is good and the reduction reaction easily proceeds actively.
In this way, in these processes, powder comprised mainly of carbon and metal oxide is shaped into green pellets and these green pellets are used as feedstock for reduction by heating
As the feedstock for the green pellets, powder ore, metal oxide dust, or other metal oxide and carbon as a reducing agent are used. In the production of reduced iron, pellet feed or other fine iron ore is used. The reducing agent used is carbon, but the ratio of the carbon not volatilizing (fix carbon) until the temperature where the reduction reaction occurs, that is, about 1100° C., is preferably a high one. As a source of such carbon, coke fines or anthracite is good.
In general, powders of two or more types of feedstock are used. This is for adjusting the ratio between the metal oxide and carbon. For production of green pellets, a pan type pelletizer is used. First, the powders of the feedstock are mixed in predetermined ratios, then the result is shaped into green pellets by the pan type pelletizer.
A pan type pelletizer is comprised of a rotary pan of a disk shape having a diameter of 2 to 6 m. The pan is inclined at about 45 degrees. A feed powder including water tumbles inside it. While this happens, the feed powder is coated around the generated nuclei which then grow into green pellets. The sufficiently grown green pellets leave the pan by their own weight.
When the rotary furnace is a rotary kiln, the green pellets are fed into the furnace without drying. This is because the temperature of the feed part of the rotary kiln is about 300° C. and the green pellets will not burst in the moist state. On the other hand, when a rotary hearth reducing furnace, the temperature of the green pellet feed part is 1000° C. or more, so green pellets containing moisture will burst due to evaporation of the moisture and therefore the green pellets are dried before being fed to the furnace.
For the feed powder containing the metal oxide (hereinafter referred to as the “metal oxide-bearing powder”), generally an ore is used, but sometimes use is made of iron-making dust or thickener sludge generated in the process of refining in a blast furnace, converter, electric furnace, etc. or the process of rolling and processing. In particular, dust or sludge generated in the steelmaking industry includes impurities such as zinc or lead, but these evaporate along with a reduction reaction of 1200° C. or more, so this reaction is an effective means for removal of impurities. It is also used as a process for treating dust containing large amounts of impurities and is effective for recycling of metal resources.
In this way, in the process for reducing green pellets, for stable operation, a high strength of the green pellets used as the feedstock is important. For example, with a vertical shaft furnace, if the green pellets are insufficient in strength, powder generated by crumbling of the green pellets will enter between the green pellets stacked in the furnace leading to the problem of obstruction of the flow of gas, the problem of too much dust trapped in the dust trap, etc. In the case of a rotary kiln, if the green pellets are insufficient in strength, the green pellets will crumble when tumbling in the kiln—leading to the problem of the dust generated at that time sticking to the refractories and creating a dam ring. As a result, the green pellets will not pass over the dam ring and green pellets will no longer flow through the inside of the kiln.
Further, in the case of a rotary hearth reducing furnace, if the green pellets are insufficient in strength, the green pellets will crumble, and the metal oxide powder of the crumbled green pellets will accumulate on the hearth and be heated and sinter at a high temperature of 1200° C. or more. The sintered powder will bond together and will sinter and bond with the refractories of the hearth as well to stick to the hearth. The stuck powder will build up on the hearth and cause wear on the blade of the screw-type discharger discharging the reduced pellets on the hearth. As a result, the blade lifetime will become extremely short. A blade which has a lifetime of at least one year in usual operation will sometimes have to be replaced in one month. Further, along with buildup on the hearth, it will become impossible to spread the green pellets on the hearth normally. To eliminate this problem, it is necessary to cool the entire furnace and use a breaker or other machine to break up and remove the buildup on the hearth. As a result, each time, the facility will have to be idled for at least five days. This is a problem causing a major drop in the operating rate.
In this way, if the green pellets are insufficient in strength, the operation of the reduction process becomes unstable. Therefore, to solve these problems, green pellets generating little powder have to be fed. Technology for producing high strength pellets under stable conditions has therefore been sought. In particular, this demand has been acute for pellets using powder containing carbon (powder coal, coke, charcoal, etc., hereinafter referred to as “carbon-bearing powder”) as a feedstock, due to the problem of the strength being harder to raise compared with pellets made of only a metal oxide powder.
As methods of obtaining a high strength shaped article having no powder, there are the method of producing spherical green pellets by a pan type pelletizer, the method of forming briquets by shaping by a mold etc., and the method of extrusion of a type extruding a shaped article from a perforated plate. Among these, green pellets obtained by a pan type pelletizer have the merit of inexpensive production of dense, high strength green pellets. The pan type pelletization method is therefore frequently used. With the conventional pelletization method, however, the only idea was to mix a powder metal oxide and a powder carbon source to form pellets. It was not always possible to produce high strength green pellets for a rotary hearth reducing furnace or other reducing furnace.
To meet with this demand, as prior art, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-193423 proposes a method of improving the strength of green pellets by mixing in an organic binder at the time of pelletization at the pan type pelletizer. However, sufficient consideration was not given to the technology relating to the distribution of the particle size, ingredients, etc. of the feed powder and other feedstock conditions and the adjustment of moisture at the time of pelletization and other operating conditions. This method did not necessarily produce high strength green pellets. Further, green pellets for use in rotary kilns or rotary hearth reducing furnaces and produced from feed powder including coke fines etc. in a ratio of at least 5% are particularly difficult to shape. Sometimes the strength can be secured by addition of a binder, but in general the problem could not be solved by just addition of a binder.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-241125 discloses an apparatus for feeding dried green pellets to a rotary furnace from a pelletizer through a pellet dryer. This is an apparatus which dries the green pellets of the feedstock in advance to prevent the green pellets from bursting due to moisture on the high temperature hearth and is important technology. A method of production of high strength green pellets which will not crumble when passed through the steps explained above and the configuration of a facility for the same have not yet however been elucidated.
The problems when pelletizing feed powder including a carbon-bearing powder are not limited to the strength of the green pellets. When the feedstock conditions are poor, there is also the problem of discontinuous ejection of green pellets from the pan type pelletizer. That is, when the distribution of the particle size of the feed powder is poor or when adjustment of the moisture is improper, the growth of the green pellets in the pelletizer becomes unstable and the green pellets alternately are almost not ejected at all from the pelletizer and are ejected in large quantities. As a result, the feed of the green pellets to the reducing furnace connected to the downstream steps of the pelletizer becomes discontinuous and the problem of the reduction reaction becoming unstable arises. Further, the reduction in strength of the green pellets at the time when this phenomenon occurs is also a major problem.
Further, even if the strength of the green pellets is high, if the green pellets are handled unsuitably, the green pellets will be broken and powder will be generated during the screening or drying operation or during transport. Therefore, handling so as to prevent crumbling of the green pellets is also an important technology. In the conventional methods, however, this fact was not given sufficient attention. In the worst cases, 20 to 30% of the green pellets crumbled into powder during transport or drying.
Further, the crumbling of the green pellets due to drying at the time of feed into the hearth is also a problem. The green pellets produced by a pan type pelletizer are dense and high in strength in the moist state, but fall in strength when dried. Therefore, prevention of crumbling is important when dropping the dried green pellets on the hearth, but sufficient care has not been taken regarding this point either.
In this way, in the prior art, stable pelletization of feed powder containing a carbon-bearing powder has been technically difficult. As a result, the operation of the reducing furnace has become unstable and there was the problem that efficient production of metal was not possible.
Further, in a rotary hearth reducing furnace, it is necessary to produce green pellets high in strength both in the wet state and the dry state. It is necessary to produce green pellets higher in strength than even the green pellets used for other purposes. Therefore, a new technology has been sought for stably producing high strength green pellets using a powder containing a carbon-bearing powder as a feedstock and for realizing handling preventing crumbling of the same.